1. Field of the Invention
The invention relates generally to polymer nanocomposites and methods for their preparation. The invention relates more specifically to the use of polymers and monomers that contain polar groups such as nitrile that can exfoliate layered inorganic materials such as layered silicates and optionally form thermally stable matrices with such materials.
2. Background
Advanced materials, particularly in the automotive and aerospace fields are needed that can withstand high temperatures. In response to this need a variety of compositions termed “nanocomposites” have been designed, as for example described in U.S. Pat. Nos. 6,323,270, 5,385,776 and 6,057,035. Nanocomposites generally are admixtures of individual platelet particles derived from intercalated layered silicate materials with one or more polymers. The admixture usually comprises a polymer matrix having one or more properties of the matrix polymer that is improved by addition of the exfoliated intercalate. The intercalate may be formed by increasing the interlayer spacing between adjacent silicate platelets. The increased spacing is achieved by adsorption of a silane coupling agent or an onium cation, such as a quaternary ammonium compound, having a reactive group which is compatible with the matrix polymer.
Phyllosilicates, such as smectite clays (e.g., sodium montmorillonite and calcium montmorillonite), can be treated to intercalate organic molecules between silicate layers. Furthermore, the organic molecules may be bonded to polymer between the layers, thereby substantially increasing the interlayer (interlaminar) spacing between the adjacent silicate layers. The treated, intercalated phyllosilicates, having interlayer spacings of at least about 10–20 Å and often up to about 100 Å, can then be exfoliated, (i.e. separate the silicate layers). This separation may be accomplished mechanically, such as by high shear mixing. Admixing individual silicate layers with a matrix polymer, before, after or during the polymerization of the polymer, can greatly contribute to one or more desirable polymer properties such as high mechanical strength and stability to high temperature, as described for example in U.S. Pat. Nos. 4,739,007; 4,810,734; and 5,385,776.
Exemplary known composites, also called “nanocomposites,” are disclosed in the published PCT disclosure of Allied Signal, Inc. WO 93/04118 and U.S. Pat. No. 5,385,776. These publications disclose admixtures of individual platelet particles derived from intercalated layered silicate materials with a polymer to form a polymer matrix having one or more properties of the matrix polymer and improved by the addition of the exfoliated intercalate. As disclosed in WO 93/04118, the intercalate is formed (the interlayer spacing between adjacent silicate platelets is increased) by adsorption of a silane coupling agent or an onium cation, such as a quaternary ammonium compound, having a reactive group that is compatible with the matrix polymer. Such quaternary ammonium cations are well-known to convert a highly hydrophilic clay, such as sodium or calcium montmorillonite, into an organophilic clay capable of sorbing organic molecules.
Nanocomposites may be formed by direct intercalation of polystyrene and poly(ethylene oxide) in organically modified silicates with a solvent as described by R. A. Vaia, et al., Chem. Mater., 5:1694–1696 (1993) and R. A. Vaja et al. Adv. Materials, 7:154–156 (1985). This intercalation may be accompanied by water displacement from between the clay platelets. Unfortunately however, the intercalated material reported in these studies was not easily exfoliated but was tested in pellet form.
More recently, higher temperature inorganic nanocomposites have been described, as explained in U.S. Pat. No. 6,057,035 issued to Singh and Haghighat. Although apparently an improvement, the use of synthetic organically modified layered silicates described in this patent generally remains limited, in most instances to temperatures well below 400 degrees centigrade and may require synthesis of a new type of alkyl group compatibility agent. A better solution would exploit known chemistry for making high temperature nanocomposites.
Presently phthalonitrile resins often are used with alternative curing agents to address the need for high temperature composites as described for example in U.S. Pat. Nos. 5,292,854 and 4,408,035. While these resins offer high-use temperatures and good fire resistance, their widespread use is restricted by microcrack formation and poor long-term thermo-oxidative stability. Both of these problems compromise mechanical properties of these materials and limit their use. These problems are alleviated by blending the resin with a conventional thermoset resin(s) such as an epoxy or imide. Unfortunately however, while this blending toughens the phthalonitrile resin this is accompanied by lower temperature stability compared to resins made from the neat phthalonitrile systems, as described in U.S. Pat. Nos. 5,939,508 and 5,132,396. Accordingly, improved materials having higher temperature resistance with good mechanical strength are needed. In particular, methods are required for reducing microcracking and increasing material thermooxidative stability, while retaining the inherent low viscosity and processability of the systems such as those made from phthalonitrile.